where is X-axis mirror in 2.8 blender
Now you've got all sorts of mirrors in the top panel

upd: I was a little bit too hurry and didn't check if mirror options for mesh edit mode are the same for armature edit mode, but x-axis mirror is still there for armature. And it works as a mirror i.e. I move left wrist and right wrist is moved symmetrically. I've added another pic. You should have the same panel in armature edit mode unless you have manually edited your default UI.